1. Field of the Invention
The present invention relates to a rotational vibration testing apparatus which is applicable to assessing the rotational vibration of a rotating object such as, for example, a magnetic disk and the like.
2. Description of the Related Art
In recent years, with magnetic disk drives, the density of tracks per inch is increasing, and the speed in seeking is becoming faster and faster, as a result of which it becomes important to enhance the accuracy of positioning a magnetic head. On the other hand, a rotary actuator is used as a rotating drive source for such magnetic disk drives, and such an actuator has a weak structure against rotational external disturbances, i.e., vibrations or oscillations in a direction in which a magnetic disk is driven to rotate. The rotational disturbances increasingly affect the positioning accuracy in an adverse manner.
In these circumstances, in the development of magnetic disk drives, it is essential to perform assessment against rotational vibrations thereof and take effective and satisfactory measures to cope with such a problem. Thus, it is highly desirable to provide a small-sized rotational vibration testing apparatus with a wide band of measuring frequencies.
Conventionally, as described in U.S. Pat. No. 5,644,087 for example, a rotational vibration testing apparatus has been proposed in which a unidirectional vibration produced by a rectilinear vibration generator is converted into a rotational vibration and transmitted to a rotary table by means of a link mechanism, so that the rotary table is thereby forced to vibrate or oscillate about an axis of rotation thereof.
Since the conventional rotational vibration testing apparatus is constructed to convert the rectilinear reciprocating vibration generated by the rectilinear vibration generator into a rotational reciprocating vibration by means of the link mechanism, as described above, there arises a problem in that the measuring frequency band is limited due to the resonance of the link mechanism.
Moreover, a combination of the rectilinear vibration generator and the link mechanism results in another problem in that the size of the apparatus is enlarged, substantially reducing transportability thereof.
The present invention is intended to obviate the above-mentioned problems, and has for its object to provide a small-sized rotational vibration testing apparatus in which the torque generated by a vibration applying source is directly transmitted to a rotary table while omitting a link mechanism conventionally required, and which has a wide band of measuring frequencies.
Bearing the above object in mind, according to a first aspect of the present invention, there is provided a rotational vibration testing apparatus comprising: a base; a rotary table rotatably supported on the base through a shaft; and a vibration applying mechanism for driving the rotary table to oscillate or vibrate in a rotational direction about the axis of the shaft which acts as an axis of rotation. The vibration applying mechanism comprises: a magnetic field generating unit for generating magnetic fields; a coil unit arranged to traverse the magnetic fields generated by the magnetic field generating unit, and a power supply unit for supplying current to the coil unit while alternately changing the direction of flow of the current. One of the magnetic field generating unit and the coil unit is mounted on the base, and the other thereof is mounted on the rotary table.
With this arrangement, there is no need for a conventional link mechanism, so it is possible to avoid the limitations on the band of measuring frequencies due to the characteristics of such a link mechanism, thus making it possible to provide a small-sized rotational vibration testing apparatus having a wide band of measuring frequencies.
In a preferred form of the invention, the rotary table comprises: a mounting plate having a first surface for mounting thereon of an object to be tested and a second surface on the opposite sides thereof; and the shaft mounted upright on the second surface of the mounting plate and having an axis thereof disposed in coincidence with a center of the mounting plate, the shaft being rotatably supported on the base. The coil unit is mounted on either one of the mounting plate and the shaft.
With this arrangement, mounting the coil unit on the rotary table serves to reduce the weight of the rotary table and transmit a magnetic force created by the coil unit to the rotary table in an efficient manner in comparison with the case where the magnetic field generating unit is mounted on the rotary table.
In another preferred form of the invention, the coil unit is attached to a coil frame unit which is mounted on either one of the mounting plate and the shaft.
This arrangement serves to improve the assemblability of the coil unit.
In a further preferred form of the invention, the coil unit comprises a plurality of coils attached to the coil frame unit.
Preferably, the coil frame unit comprises a single coil frame. The coil frame may include an annular engaging body adapted to be fixedly mounted on the outer periphery of the rotary table; and a plurality of coil mounting portions disposed on the outer periphery of the annular engaging body at an equal circumferential pitch and extending radially outward from the annular engaging body.
Preferably, the annular engaging body and the plurality of coil mounting portions are formed integral with each other. Preferably, the coil frame unit comprises a plurality of coil frames to which the coils can be respectively attached, the coil frames being adapted to be fixedly mounted on the outer periphery of the rotary table.
Thus, attaching the coils to the coil frame unit serves to ensure a proper positional relation between the coils, enhancing the assemblability of the coils.
In a still further preferred form of the invention, the rotational vibration testing apparatus further comprises a positioning engagement portion provided on the mounting plate for positioning the coil frames.
Preferably, the positioning engagement portion comprises at least one groove or step formed on the outer peripheral portion of the rotary table, the plurality of coil frames being adapted to be fitted into the at least one groove whereby they are mounted on the rotary table.
This arrangement serves to ensure a proper mutual positional relation between the coils and the rotary table, contributing to improvements in the assemblability thereof.
In a yet further preferred form of the invention, the magnetic field generating unit comprises a plurality of magnetic field generators, and the coil unit comprises a plurality of coils, wherein the magnetic field generators and the coils are arranged in pairs at an equal circumferential pitch about the axis of rotation.
With this arrangement, magnetic forces generated by the coils when power is supplied to the coils are transmitted to the rotary table in a well-balanced manner, so there takes place stable rotational vibration in a rotational direction of the, rotary table.
In a further preferred form of the invention, the rotational vibration testing apparatus further comprises a position returning mechanism for returning a positional relation between the coil unit and the magnetic field generating unit to a predetermined positional relation when power supplied to the coil unit is stopped.
Preferably, the position returning mechanism comprises a resiliently operated mechanism for resiliently generating a returning force for returning the positional relation between the coil unit and the magnetic field generating unit to the predetermined positional relation.
Preferably, the position returning mechanism comprises a magnetically operated mechanism for magnetically generating a returning force for returning the positional relation between the coil unit and the magnetic field generating unit to the predetermined positional relation.
Preferably, the rotational vibration testing apparatus further comprises a cooling unit for cooling the coil unit.
With the above arrangement, it is possible to prevent damages such as breakage of the magnetic field generating unit and/or the coil unit resulting from possible deviations in the position of the coil unit.
In a further preferred form of the invention, the rotational vibration testing apparatus further comprises a current feed-back circuit for feeding back a coil current supplied to the coil unit so as to control the coil current to a constant level.
With this arrangement, the current flowing through the coil unit becomes constant, so the coil unit is able to generate a constant force, thereby giving rise to rotational oscillation or vibration with a constant acceleration. Thus, the apparatus can be applied to high precision tests.
In a further preferred form of the invention, the rotational vibration testing apparatus further comprises an acceleration sensor for sensing an acceleration of the rotary table, wherein an output of the acceleration sensor is fed back so as to control a coil current supplied to the coil unit to thereby make the acceleration of the rotary table to a constant value
With this arrangement, even if the testing apparatus happens to resonate, it can nevertheless generate oscillation or vibration of a constant acceleration, as a result of which the apparatus is applicable to tests of still higher precision. In a further preferred form of the invention, the rotational vibration testing apparatus further comprises: an arm protruded from the rotary table; and a stop adapted to engage the arm for limiting a movement of the arm and hence of the rotary table. The arm and the stop are arranged such that when the rotary table is driven to rotate to an excessive extent so as to engage the stop by means of the vibration applying mechanism.
This arrangement can provide the rotational vibration testing apparatus with an impact testing function.
Preferably, the coil unit comprises a plurality of coils electrically connected in series with each other.
Preferably, the coil unit comprises a plurality of coils electrically connected in parallel with each other.
Preferably, the coil unit comprises a plurality of coils electrically connected with each other in a combination of series and parallel connections.
The above and other objects, features and advantages of the present invention will be more readily apparent to the skilled in the art from the following detailed description of presently preferred embodiments of the invention taken in conjunction with the accompanying drawings.